Anti-forgery/anti-alteration measures are important elements for the printed products of important documents including various kinds of certifications and securities such as stock certificates and bonds. To prevent forgery or alteration of such printed products, a method using, as a design, a figure containing many geometrical patterns and a method of visualizing an unrecognizable latent image by applying some means and effect to a printed product are mainly available.
A representative example of the former method uses a geometrical pattern such as background pattern, a guilloche design pattern, or a relief pattern which are widely used as a design of a securities printed product. In the anti-forgery/anti-alteration measures using a geometrical pattern, a pattern is formed by an aggregate of curved image lines (in the present invention, curved image lines include straight image lines) which fundamentally have a predetermined line width.
These patterns apply artistry to a printed product. The anti-forgery effect is enhanced by, e.g., extracting a pattern by photoengraving or using, in a pattern, colors which are hard to reproduce by a copy machine or complex curved image lines which generate moiré on a scanning line input/output of a copying machine or scanner. However, recently coming advanced photoengraving apparatuses or copying machines render the anti-forgery/anti-alteration measures ineffectual.
The present inventors have proposed the following techniques (1) and (2).
(1) The present inventors have filed an application of an anti-copy pattern forming method and a printed product (Japanese Patent Application No. 6-206140) characterized in that in an aggregate pattern of curved image lines, a portion having no latent image is expressed by a single image line while a portion having a latent image is expressed by at least two image lines. The total line width of the at least two image lines of the portion with a latent image equals the line width of the single image line of the portion having no latent image. The at least two image lines branch from the single image line of the portion without a latent image. The boundary line on the image lines between the portion without a latent image and the portion with a latent image is a straight line which almost perpendicularly crosses a straight line contacting a basic curve of the aggregate pattern of curved image lines at the intersection between the basic curve and the outline of the latent image.
(2) The present inventors have filed an application of a printed product (Japanese Patent Application No. 7-138879) in which in an aggregate pattern of curved image lines, a portion having no latent image is expressed by a solid line while a portion having a latent image is expressed by a periodically broken line. In one period of the periodically broken line of the portion with a latent image, including an actually printed image line portion and a non-image line portion where the image line breaks so no image line exists, the area of the non image line portion is added to that of the image line portion. The image line area ratio of the portion with a latent image and that of the portion without a latent image are the same in the same length in the curve direction.
There are provided an anti-copy pattern forming method and a printed product, which cause a printed product having a pattern according to (1) or (2) to apply an effect of preventing forgery and alteration by a copying machine to an aggregate pattern of curved image lines such as background pattern, a guilloche design pattern, or a relief pattern on important documents including various kinds of certifications and securities such as stock certificates and bonds which require anti-copy.
However, the anti-copy measure using the above-described technique (1) or (2) cannot be a sufficient anti-forgery measure any more because of advanced color copying machines and advanced DTP (DeskTop Publishing) technology.
To solve this problem, a machine read check method capable of a mass authenticity determination process at a high speed has been proposed. However, current machine read check methods of checking a printed product detect functional inks such as magnetic ink, infrared reflection/absorption ink, or fluorescent ink, or materials such as fibers, materials, and chemicals of printing media. These technologies are based on, e.g., specific electromagnetic waves imperceptible to a human and can be applied to only economically appropriate products from the viewpoint of production cost because many technologies depend on the material properties of printed products.
An example of a reading method without particularly considering the production cost of printed products is a method of optically reading a pattern on a printed product capable of using a printing material such as a general print ink. As relatively easy optical reading methods, OCR, OMR, barcode, and two-dimensional code are known. However, to use these optical reading methods for existing products, it is necessary to change the design and specifications.
These optical reading methods are popular in the market but insufficient as an anti-forgery/anti-alteration measure because any symbol is visible as a printed image line and may be decoded and altered.
There are a series of technologies generally called an electronic watermark, which are included in the optical reading methods and apply read information without changing the artistry of, e.g., a design. The electronic watermark is also called a concealed image or digital watermark. As a main application purpose, the electronic watermark embeds copyright information in a document file or a printed product thereof in an advanced copy technology or DTP technology.
The electronic watermark is said to be able to suppress degradation in frequency characteristic even in a replica. Recently, digital images distributed on the Internet often contain an electronic watermark for the purpose of copyright protection. The electronic watermark also takes effect even on a printed product and is often used in posters.
The electronic watermark can maximize its effect in a continuous tone (photo tone) pattern. A continuous tone (photo tone) pattern is multilevel image data and therefore has sufficient redundancy. Many techniques such as a pixel substitution type, pixel space using type, and quantization error diffusion type are proposed in addition to a frequency using type. There are also a lot of references and patent applications related to this technique which is one of the technologies that have received attention presently.
However, an aggregate pattern of curved image lines such as background pattern, a guilloche design pattern, or a relief pattern used in securities is basically a binary image. For this reason, the redundancy is low, and it is difficult to embed an electronic watermark. This poses problems of a low read signal strength and low read accuracy.
Hence, there is a demand for development of an anti-forgery/anti-alteration technology which is independent of the material properties of a printed product and serves as an effective technology of determining authenticity, by a machine reading method, a pattern having anti-forgery properties suitable for important documents including various kinds of certifications and securities such as stock certificates and bonds.